ece1750, spring 2018 week 5 – mosfet gate driversakwasins/power electronics week 4...
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ECE1750, Spring 2018
Week 5 – MOSFET Gate Drivers
1
Power MOSFETs(a high-speed voltage-controlled switch)(a high-speed, voltage-controlled switch)
D: DrainD: DrainIf desired, a series blocking diode can be inserted here to prevent reverse current
D
G: Gate
Switch closes when VGS ≈ 4Vdc
G
S: Source
N channel MOSFET equivalent circuit
e GS dcS
• Controllability? - Controlled turn on, controlled turn off (but there is an internal antiparallel diode). Thanks to the diode it can conduct in both directions but it cannot block D-S voltages in which VD<VS.
2
g
• Controlled through the gate by voltage. If VGS>VGS,th it conducts. Otherwise, it does not conduct (in the forward direction).
We Avoid the Linear (Lossy) Region, Using We Avoid the Linear (Lossy) Region, Using Only the On and Off States
Ideal MOSFET “off”
Ideal MOSFET “on”
DD
SS
when VGS = 20 V when VGS = 0 V
3
Power MOSFETSMain characteristics• Static Characteristic
Ohmic
i
RegionON State
Active Region
iD iD
Higher
ON State
gVGS
v
Lower VDS
4
vDS vGSCutoff (OFF State)VGS<VGS,th
VGS,th
Main characteristics – Behavioral modelsPower MOSFETS
• Static model
ON h VGS>VGS h(sw) sw =
ON when VGS>VGS,thOFF when VGS<VGS,th
• Dynamic ModelD
Capacitances (particularly Cgs) need to be charged or
rd(T)
C d
iD
D g ) gdischarged to turn the MOSFET ON or OFF,
respectively
RDS,ONCgs
CgdCds
G
5S
We Want to Switch Quickly to Minimize Switching LossesTurn Off Turn On
VDS(t)
Turn Off
VDS(t)
Turn On
I(t) I(t)∆toff ∆ton
0 0
I(t) I(t)∆toff ∆ton
pLOSS(t) pLOSS(t)
0 0
Energy lost per turn off
Energy lost per turn on00
6
Turn off and turn on times limit the frequency of operation because their sum must be considerably less than period T (i.e., 1/f)
Consider, for example, the turn off
VDS(t)
Turn Off
VEnergy lost per turn off is proportional to
0
V • I • ∆toff ,
so we want to keep turn off (and turn on) times as small
I(t)
I
(and turn on) times as small as possible.
The more often we switch, the more
p OSS(t) ∆toff
0“energy loss areas” we experience per second.
Thus, switching losses (average W) pLOSS(t) ∆toff
0Energy lost per turn off
, g ( g )are proportional to switching frequency f, V, I, ∆toff, and ∆ton.
7And, of course, there are conduction losses that are equal to the product of RDSon and I squared
Switching losses• With a resistive load, voltage and current waveforms are approximately
like (linear commutation)
turn-on losses: turn-off losses:
offVI ffV I
Total switching losses:
( ) offonon off
on on
VIp t t V tt t
( ) off onoff on
off off
V Ip t t I tt t
0 0
1 ( ) ( ( ) ( )) ( )6 6
T T on off on offsw on off on off sw
I V I VP p t dt f p t p t dt f t t f t
T
Switching losses• With in inductive load and an ideal switch, voltage and current
waveforms are approximately likeMore realistic behavior
Approximate behavior
turn-on losses: turn-off losses:
offV I
Total switching losses:
( ) 0onp t , ,
( )( / 2) ( / 2)
off onoff on off
L off L off
Ip t I t V tt t
,0
1 ( ) ( )2 2
T on off on offsw L off sw
I V I VP p t dt f t f t
T
Advantages of Operating Above 20kHzYes switching losses in power electronic switches do increase withYes, switching losses in power electronic switches do increase with operating frequency, but going beyond 20kHz has important advantages. Among these are
• Humans cannot hear the circuits
• For the same desired smoothing effect, L’s and C’s can be smaller because, as frequency increases and period T decreases, L’s and C’ h d di h l l f ti
• Humans cannot hear the circuits
C’s charge and discharge less energy per cycle of operation. Smaller L’s and C’s permit smaller, lighter circuits.
• Correspondingly, L and C rms ripple currents decrease, so current p g y, pp ,ratings can be lower. Thus, smaller, lighter circuits.
• AC transformers are smaller because, for a given voltage rating, the peak flux density in the core is reduced (which means transformerpeak flux density in the core is reduced (which means transformer cores can have smaller cross sectional areas A).
)cos()sin()( maxmax tNABtBdNAdBNAdNtv
10
)cos()( max tNABdt
NAdt
NAdt
Ntv
Thus, smaller, lighter circuits. N = number of turns, ϕ = magnetic flux, B = magnetic field, A = x-sectional area
Fast Switching Frequencies• Previous slide was just to illustrate how, with increased
switching frequency, one can reduce the size of AC transformer cores needed:transformer cores needed:
)cos()sin()( maxmax tNAB
dttBdNA
dtdBNA
dtdNtv
dtdtdt
Thus, smaller, lighter circuits. N = number of turns, ϕ = magnetic flux, B = magnetic field, A = x-sectional area
• The drawback, of course, to high frequency switching is increased power loss, since: P (loss) = P x number of switching eventsPTotal (loss) = Pswitching loss x number of switching events
(or, the switching frequency)
• This is the downside of high-frequency switching. Thus, one must work to ensure overall losses are reduced by working to reduce the individual switching transition time. 11
• MOSFETS are usually controlled with a pulse-width modulation (PWM) strategy.
• In essence, the PWM process involves comparing a sawtooth (or any other , p p g ( yperiodic function with linear transitions) with a voltage level. If the voltage level is constant, it will tend to produce a constant (dc) output.
Sawtooth or triangleadjustable analog input (duty cycle control)
Linear portions of sawtooth allows to directly translate voltage levels into time intervals (on-times)
12V
Gate Driver
( TC1427)(e.g. TC1427)
12Comparator
(e.g., LM393)
• PWM
D = 8/12 = 0 75D 8/12 0.75
Gate Driver
( TC1427)(e.g. TC1427)
13Comparator
(e.g., LM393)
• PWM
D = 6/12 = 0 5D 6/12 0. 5
Gate Driver
( TC1427)(e.g. TC1427)
14Comparator
(e.g., LM393)
• PWM
D = 4/12 = 1/3D 4/12 1/3
Gate Driver
( TC1427)(e.g. TC1427)
15Comparator
(e.g., LM393)
MOSFETS are Very Sensitive to Static Electricity
• Touching the gate lead before the MOSFET is properly mounted will likely ruin the MOSFET
• But it may not fail right away. Instead, the failure may be gradual. Your circuit will work, but not correctly. Performance gradually deteriorates.
• When that happens, you can spend unnecessary hours debugging
• Key indicators of a failed MOSFET are
• Failed or burning hot driver chip• Burning hot gate driver resistor (discolored or bubbled up)Burning hot gate driver resistor (discolored, or bubbled up)
• Board scorches or melts underneath the driver chip or gate driver resistor
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Avoid these problems by mounting the MOSFET last, by using an antistatic wristband, and by not touching the gate lead
MOSFET Switch Turn-Off Overshoot..200kHz, 0.01µF snubber
VDSSimple snubbercircuit
200kHz no snubber 100kHz 0 01µF snubber
VGS
200kHz, no snubber 100kHz, 0.01µF snubber
VDS VDS
VGS VGS
200kHz, 0.0022µF snubber 50kHz, 0.01µF snubber
VDS VDS
17VGS VGS
MOSFET Safe Operating Area (SOA)
Pulsed drain current must never be exceed
Maximum continuous drain
current can be exceeded but onlymust never be exceed exceeded but only
for a brief time
Thermal limit (power limit) can be exceeded but only for a brief timeOperation naturally
limited by RDS on
Breakdown voltage must never be
limited by RDS,on (Ohm’s law)
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must never be exceeded
MOSFET Datasheet
19
Small 10Ω , 100W Resistor. 22Vdc. Vpeak = 240V.
MOSFET opens
VDS
VGS
ON
OFF
DT
20Note: Ringing is caused by the interaction of parasitic capacitors and inductors.
DT
Small 10Ω , 100W Resistor. 22Vdc. Vpeak = 40V.
0 022 S C0.022µF Snubber Cap
MOSFET opens
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Small 10Ω , 100W Resistor. 22Vdc. Vpeak = 60V
0 0068 S C0.0068µF Snubber Cap
MOSFET opens
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Gate Drivers
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Integrated PWM
TT CRf
2.1
MC34060A, Fixed Frequency, PWM, Voltage Mode Single Ended Controller
241
T Tf
C R